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Overview

The visitor pattern enables traversal and interpretation of the Abstract Syntax Tree (AST) produced by the parser. The Expresiones compiler uses two visitor classes:
  1. ExpresionesVisitor - Base visitor class generated by ANTLR (provides default traversal)
  2. Visitor - Custom implementation that performs semantic analysis and interpretation

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Base Visitor Class (ExpresionesVisitor)

Generated by ANTLR from Expresiones.g, this class provides empty visit methods for all grammar rules. Location: ~/workspace/source/ExpresionesVisitor.py 
from antlr4 import *
if "." in __name__:
    from .ExpresionesParser import ExpresionesParser
else:
    from ExpresionesParser import ExpresionesParser

class ExpresionesVisitor(ParseTreeVisitor):
    # Visit methods for all grammar rules...

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Available Visit Methods

From ExpresionesVisitor.py:12-84: 
def visitProg(self, ctx:ExpresionesParser.ProgContext):
    return self.visitChildren(ctx)

def visitInstrDecl(self, ctx:ExpresionesParser.InstrDeclContext):
    return self.visitChildren(ctx)

def visitInstrAsig(self, ctx:ExpresionesParser.InstrAsigContext):
    return self.visitChildren(ctx)

def visitInstrIf(self, ctx:ExpresionesParser.InstrIfContext):
    return self.visitChildren(ctx)

def visitBloque(self, ctx:ExpresionesParser.BloqueContext):
    return self.visitChildren(ctx)

def visitDeclaracion(self, ctx:ExpresionesParser.DeclaracionContext):
    return self.visitChildren(ctx)

def visitAsignacion(self, ctx:ExpresionesParser.AsignacionContext):
    return self.visitChildren(ctx)

def visitRelacional(self, ctx:ExpresionesParser.RelacionalContext):
    return self.visitChildren(ctx)

def visitParentesisCond(self, ctx:ExpresionesParser.ParentesisCondContext):
    return self.visitChildren(ctx)

def visitNotLogica(self, ctx:ExpresionesParser.NotLogicaContext):
    return self.visitChildren(ctx)

def visitLogica(self, ctx:ExpresionesParser.LogicaContext):
    return self.visitChildren(ctx)

def visitNumero(self, ctx:ExpresionesParser.NumeroContext):
    return self.visitChildren(ctx)

def visitVariable(self, ctx:ExpresionesParser.VariableContext):
    return self.visitChildren(ctx)

def visitAritmetica(self, ctx:ExpresionesParser.AritmeticaContext):
    return self.visitChildren(ctx)

def visitParentesisExpr(self, ctx:ExpresionesParser.ParentesisExprContext):
    return self.visitChildren(ctx)
The base visitor simply calls visitChildren() for each node. Custom implementations override these methods to add functionality.

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Custom Visitor Implementation

The Visitor class extends ExpresionesVisitor to provide semantic analysis, symbol table management, and interpretation. Location: ~/workspace/source/Visitor.py

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Class Structure

From Visitor.py:10-12: 
class Visitor(ExpresionesVisitor):
    def __init__(self):
        self.tabla_simbolos = {}  # Symbol table: {name: Simbolo}
The visitor maintains state through a symbol table that maps variable names to Simbolo objects (see Symbol Table for details).

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Visit Method Implementations

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1. visitProg - Program Entry Point

From Visitor.py:14-16: 
def visitProg(self, ctx):
    print("Iniciando ejecución...")
    return self.visitChildren(ctx)
Called when visiting the root ProgContext. Prints a startup message and traverses all child nodes (instructions). Example: 
program {
    int x = 10;
}
Output: Iniciando ejecución...

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2. visitInstrDecl - Variable Declaration

From Visitor.py:18-31: 
def visitInstrDecl(self, ctx):
    tipo = ctx.declaracion().TIPO().getText()
    nombre = ctx.declaracion().ID().getText()
    
    valor_inicial = None
    if ctx.declaracion().ASIGNACION():
        valor_inicial = self.visit(ctx.declaracion().expr())
    
    if nombre in self.tabla_simbolos:
        print(f"Error Semántico: Variable '{nombre}' ya declarada.")
    else:
        self.tabla_simbolos[nombre] = Simbolo(nombre, tipo, valor_inicial)
        print(f"Declaración: {nombre} ({tipo}) = {valor_inicial}")
    return None
Functionality:
  • Extracts type and identifier from the declaration context
  • Evaluates initialization expression if present
  • Checks for duplicate declarations (semantic error)
  • Adds symbol to symbol table
  • Prints declaration information
Example 1: Declaration without initialization 
int x;
Output: Declaración: x (int) = None Example 2: Declaration with initialization 
float pi = 3.14;
Output: Declaración: pi (float) = 3.14 Example 3: Duplicate declaration (error) 
int x = 5;
int x = 10;
Output: 
Declaración: x (int) = 5
Error Semántico: Variable 'x' ya declarada.

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3. visitInstrAsig - Variable Assignment

From Visitor.py:33-42: 
def visitInstrAsig(self, ctx):
    nombre = ctx.asignacion().ID().getText()
    valor = self.visit(ctx.asignacion().expr())
    
    if nombre in self.tabla_simbolos:
        self.tabla_simbolos[nombre].valor = valor
        print(f"Asignación: {nombre} = {valor}")
    else:
        print(f"Error: Variable '{nombre}' no declarada.")
    return valor
Functionality:
  • Extracts variable name and evaluates expression
  • Checks if variable exists in symbol table
  • Updates variable value or reports error
  • Returns the assigned value
Example 1: Valid assignment 
int x = 5;
x = 10;
Output: 
Declaración: x (int) = 5
Asignación: x = 10
Example 2: Undeclared variable (error) 
y = 20;
Output: Error: Variable 'y' no declarada.

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4. visitAritmetica - Arithmetic Operations

From Visitor.py:44-53: 
def visitAritmetica(self, ctx):
    izq = self.visit(ctx.expr(0))  # Left operand
    der = self.visit(ctx.expr(1))  # Right operand
    op = ctx.getChild(1).getSymbol().type  # Operator token type
    
    if op == ExpresionesParser.SUMA: return izq + der
    if op == ExpresionesParser.RESTA: return izq - der
    if op == ExpresionesParser.MULT: return izq * der
    if op == ExpresionesParser.DIV: return izq // der if der != 0 else 0
    return 0
Functionality:
  • Recursively evaluates left and right expressions
  • Determines operator type from middle child
  • Performs arithmetic operation based on operator
  • Handles division by zero (returns 0)
Division uses integer division (//). For floating-point division, modify to use /.
Example: 
int result = (10 + 5) * 2;
Evaluation:
  1. visitAritmetica for +: 10 + 5 = 15
  2. visitAritmetica for *: 15 * 2 = 30
  3. Result: 30

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5. visitRelacional - Relational Comparisons

From Visitor.py:55-66: 
def visitRelacional(self, ctx):
    izq = self.visit(ctx.expr(0))
    der = self.visit(ctx.expr(1))
    op = ctx.op.type  # Operator stored in ctx.op field
    
    if op == ExpresionesParser.MAYOR: return izq > der
    if op == ExpresionesParser.MENOR: return izq < der
    if op == ExpresionesParser.IGUAL: return izq == der
    if op == ExpresionesParser.DIFERENTE: return izq != der
    if op == ExpresionesParser.MAYOR_IGUAL: return izq >= der
    if op == ExpresionesParser.MENOR_IGUAL: return izq <= der
    return False
Functionality:
  • Evaluates both expressions
  • Retrieves operator from ctx.op field (set by parser)
  • Performs comparison and returns boolean result
Example: 
if (x > 5) { ... }
If x = 10, returns True

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6. visitLogica - Logical Operations

From Visitor.py:68-74: 
def visitLogica(self, ctx):
    izq = self.visit(ctx.condicion(0))
    der = self.visit(ctx.condicion(1))
    op = ctx.getChild(1).getSymbol().type
    if op == ExpresionesParser.Y_LOGICO: return bool(izq and der)
    if op == ExpresionesParser.O_LOGICO: return bool(izq or der)
    return False
Functionality:
  • Evaluates both condition expressions
  • Performs logical AND or OR
  • Converts result to boolean
Example: 
if (x > 5 && y < 10) { ... }
Returns True only if both conditions are true

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7. visitNotLogica - Logical Negation

From Visitor.py:76-77: 
def visitNotLogica(self, ctx):
    return not self.visit(ctx.condicion())
Functionality:
  • Evaluates condition
  • Returns boolean negation
Example: 
if (!(x > 5)) { ... }
Returns True if x <= 5

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8. visitNumero - Number Literals

From Visitor.py:79-81: 
def visitNumero(self, ctx):
    val = ctx.NUMERO().getText()
    return float(val) if '.' in val else int(val)
Functionality:
  • Extracts number text from token
  • Returns float if contains decimal point, otherwise int
Examples:
  • 42 → int(42)
  • 3.14 → float(3.14)

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9. visitVariable - Variable Reference

From Visitor.py:83-88: 
def visitVariable(self, ctx):
    nombre = ctx.ID().getText()
    if nombre in self.tabla_simbolos:
        val = self.tabla_simbolos[nombre].valor
        return val if val is not None else 0
    return 0
Functionality:
  • Looks up variable in symbol table
  • Returns stored value or 0 if uninitialized/undeclared
Returning 0 for undeclared variables masks errors. Consider raising an exception instead.

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10. visitInstrIf - Conditional Statements

From Visitor.py:90-95: 
def visitInstrIf(self, ctx):
    if self.visit(ctx.condicion()):
        return self.visit(ctx.bloque(0))  # Then block
    elif ctx.bloque(1):
        return self.visit(ctx.bloque(1))  # Else block
    return None
Functionality:
  • Evaluates condition
  • Executes then-block if true
  • Executes else-block if false (when present)
Example: 
if (x > 10) {
    y = x * 2;
} else {
    y = x / 2;
}

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11. visitParentesisExpr - Parenthesized Expression

From Visitor.py:97: 
def visitParentesisExpr(self, ctx): 
    return self.visit(ctx.expr())
Functionality:
  • Simply evaluates the wrapped expression
  • Parentheses only affect parsing precedence, not evaluation

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12. visitParentesisCond - Parenthesized Condition

From Visitor.py:98: 
def visitParentesisCond(self, ctx): 
    return self.visit(ctx.condicion())
Functionality:
  • Evaluates the wrapped condition

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Visitor Pattern Flow

1

Parser Creates AST

The parser builds a tree of context objects (ProgContext, InstrDeclContext, etc.)
2

Visitor Initialization

Create a Visitor instance with empty symbol table
3

Start Traversal

Call visitor.visit(tree) with the root context
4

Visit Method Dispatch

ANTLR automatically calls the appropriate visit method based on context type
5

Recursive Traversal

Each visit method calls visit() on child contexts as needed
6

Result Propagation

Return values bubble up from leaf nodes (literals, variables) to root

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Complete Example

Source Code: 
program {
    int x = 10;
    int y = 5;
    int result = x + y * 2;
}
Visitor Execution Trace:
  1. visitProg() - Prints “Iniciando ejecución…”
  2. visitInstrDecl() - Declaration: x (int) = 10
  3. visitInstrDecl() - Declaration: y (int) = 5
  4. visitInstrDecl() - Evaluates x + y * 2:
    • visitVariable('x') → 10
    • visitVariable('y') → 5
    • visitNumero('2') → 2
    • visitAritmetica('*') → 5 * 2 = 10
    • visitAritmetica('+') → 10 + 10 = 20
    • Declaration: result (int) = 20
Output: 
Iniciando ejecución...
Declaración: x (int) = 10
Declaración: y (int) = 5
Declaración: result (int) = 20

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Extending the Visitor

To add new functionality:
1

Identify Grammar Rule

Determine which grammar rule(s) need new behavior
2

Override Visit Method

Override the corresponding visit method in the Visitor class
3

Access Context Data

Use context methods to access tokens and child nodes
4

Implement Logic

Add semantic analysis, code generation, or interpretation logic
5

Return Value

Return appropriate value for parent visit methods
def visitInstrAsig(self, ctx):
    nombre = ctx.asignacion().ID().getText()
    valor = self.visit(ctx.asignacion().expr())
    
    if nombre in self.tabla_simbolos:
        simbolo = self.tabla_simbolos[nombre]
        
        # Type checking
        if simbolo.tipo == 'int' and not isinstance(valor, int):
            print(f"Error de Tipo: Se esperaba 'int', se obtuvo '{type(valor).__name__}'")
            return None
        elif simbolo.tipo == 'float' and not isinstance(valor, (int, float)):
            print(f"Error de Tipo: Se esperaba 'float', se obtuvo '{type(valor).__name__}'")
            return None
        
        simbolo.valor = valor
        print(f"Asignación: {nombre} = {valor}")
    else:
        print(f"Error: Variable '{nombre}' no declarada.")
    return valor
See also:
  • Symbol Table - Variable storage and management
  • Parser - Context classes created by the parser

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